What makes a phar a phar and not a tar or a zip?
PHP Manual

Phar manifest file entry definition

Each file in the manifest contains the following information:

Phar Manifest file entry
Size in bytes Description
4 bytes Filename length in bytes
?? Filename (length specified in previous)
4 bytes Un-compressed file size in bytes
4 bytes Unix timestamp of file
4 bytes Compressed file size in bytes
4 bytes CRC32 checksum of un-compressed file contents
4 bytes Bit-mapped File-specific flags
4 bytes Serialized File Meta-data length (0 for none)
?? Serialized File Meta-data, stored in serialize format

Note that as of API version 1.1.1, empty directories are stored as filenames with a trailing slash like my/directory/

The File-specific bitmap values recognized are:

Bitmap values recognized
Value Description
0x000001FF These bits are reserved for defining specific file permissions of a file. Permissions are used for fstat and can be used to recreate desired permissions upon extraction.
0x00001000 If set, this file is compressed with zlib compression
0x00002000 If set, this file is compressed with bzip compression